1. Field of the Invention
This invention relates to improvements in devices for alignment and focusing of a fundus camera.
A fundus camera is used to capture an image of the fundus (posterior portion of the eye, including the retina and blood vessels). This fundus image can be examined by a third party or compared to fundus images taken at an earlier date, to facilitate diagnosis of retinal disease.
The fundus camera comprises an objective lens to form an image of the fundus of the eye, other lenses to form an image on the recording plane of a camera, and an illumination system to project light into the eye so that the fundus image is sufficiently bright that an image may be captured on the camera.
To enable a useful image to be formed without unacceptable reflections from the anterior parts of the eye (including the cornea, iris and vitreous lens) the fundus camera must be aligned very accurately. For capture of fundus images without mydriasis to dilate the pupil, the required accuracy of the fundus camera alignment is difficult to achieve without features to guide the operator in the alignment of the fundus camera. Once correct the fundus camera alignment has been achieved, that alignment must be maintained while the camera focus is adjusted.
2. Description of the Related Art
To facilitate accurate focusing of the fundus camera, one or more lines may be projected onto the eye fundus and the reflections of these lines may then be observed through the fundus camera.
Further, split prisms may be provided within the illumination system such that the image of the projected lines on the eye fundus appears split if the camera is not correctly focused, as discussed in U.S. Pat. No. 3,925,793.
The optical components for such a system for projecting a line to aid focusing may be separate from the illumination system and combined with an afocal lens system, such that the focus of the viewing system and the focal plane of the projected line system may be maintained coincident as the focus of the viewing system is adjusted as discussed in U.S. Pat. No. 4,187,014.
If the fundus camera alignment changes during focusing, for example due to movement of the subject's eye, then the image captured may be degraded by loss of light and reflections from the anterior portion of the eye. Thus it is important that the alignment aids be available to the operator while focus is being adjusted.
Previous inventions have provided for one or more alignment marks that may be projected such that they form a single focused image in the camera system when reflected from the cornea of the eye, such as U.S. Pat. No. 4,251,139 et al.
This image may be superimposed on the image of the eye fundus, thus allowing the operator to monitor the alignment of the fundus camera while adjusting focus of the fundus image. The operation of a fundus camera incorporating such alignment marks is not intuitive and skill is required by the operator to interpret the image seen through the fundus camera to allow correct alignment of the fundus camera. Also the focus of the alignment marks may change as the focus of the fundus camera is adjusted to suit the particular subject's eye.
Alternatively, the alignment of the fundus camera can be facilitated by providing the operator with an in-focus view of the anterior portions of the eye, including the iris, during alignment. Such view has been found to not give sufficient information for accurate alignment, particularly in the axial direction, so that an additional indication of axial position of the fundus camera is desirable.
Previous inventions have provided for projection of 2 separate alignment marks onto the iris of the eye to facilitate alignment. The alignment marks may be formed by infrared light, such that the pupil of the eye does not constrict if the light enters the subject's eye. When the fundus camera is correctly positioned, the two alignment marks are coincident and in focus on the iris of the eye as discussed in U.S. Pat. Nos. 4,257,688 and 4,252,420 and 4,253,743.
Correct alignment of the fundus camera using such alignment marks has been found to be intuitive and straightforward. An observation system must be provided, however, to enable the alignment marks to be observed by the operator. This observation system may incorporate a camera system that is separate from the fundus camera optical path, but this increases the bulk of the fundus camera in proximity to the subject and the field of view of the observation system may be obscured by the subject's facial features.
To avoid these problems, the observation system may share part of the fundus camera optical path including the objective lens, by including optical components into that system so as to enable an image of the iris part of the eye to be formed. This observation system may include an infrared camera.
Previous inventions have provided for the formation of an image of anterior parts of the eye including the iris for fundus camera alignment by interposing an auxiliary lens or lenses behind the objective lens such as U.S. Pat. No. 3,936,844.
This auxiliary lens would allow the alignment marks on the iris of the eye to be observed, but does not allow the fundus of the eye to be observed concurrently.
Fundus cameras currently available that provide an anterior view option for alignment typically require the operator to manually switch the instrument from anterior view mode for alignment to fundus view mode for focusing. It is desirable that both the anterior and posterior portions of the eye are visible concurrently to facilitate intuitive, easy and accurate alignment of the fundus camera and correct focus adjustment. Also, it is desirable that focusing marks be projected near to the centre part of the fundus field of view for optimum focussing.